Method of and apparatus for controlling plasma potential and eliminating unipolar arcs in plasma chambers

ABSTRACT

A method of and apparatus for controlling the potential of a plasma including a metal-walled chamber and a conductive coil which carries a radio-frequency current and is wrapped around the metal-walled chamber to produce a plasma within the chamber. A filament made of refractory metal has two ends, and a central portion formed in the shape of a probe. The central portion of the filament extends into the interior of the chamber and the two ends of the filament pass through a wall of the chamber to the exterior of the chamber. A heating power supply is connected to the two ends to the filament and to the chamber wall for heating the filament to a predetermined temperature above that of the plasma. The heated filament produces thermionic emissions from the filament to the plasma in order to control the plasma potential and eliminate unipolar arcing at the chamber wall.

FIELD OF INVENTION

The present invention relates to a CVD coating apparatus, and moreparticularly to a CVD (chemical vapor deposition) coating apparatus andmethod using a metal-walled plasma chamber.

BACKGROUND

When a conductor loop or coil carrying radio frequency current iswrapped around a dielectric chamber containing gas at low pressure, aplasma is formed inside the chamber as a result of currents induced bythe changing magnetic field of the coil known as inductive coupling.Plasmas may be produced inside metal-walled chambers, if the chambersare suitable for high power CVD diamond reactors, which is the subjectof a recent patent application U.S. Ser. No. 08/483,982, to A. E.Robson, et al, for a Durable Plasma Treatment apparatus and Method.

Although the primary advantage of metal chambers is to allow much graterpower to be dissipated in the plasma than is possible in dielectricchambers, there is a secondary advantage to slotted metal chambers: thechamber acts as a Faraday cage and prevents capacitive coupling betweenthe coil and the plasma. As is well known, capacitive coupling can leadto significant potentials arising between the coil and the plasma. As iswell known, capacitive coupling can lead to significant potentialsarising between the plasma and the wall of a dielectric chamber, and theresulting acceleration of plasma ions toward the walls can bedeleterious to many processes.

In the case of metal walled chambers, the maximum potential between theplasma and the wall is limited to the "floating potential," V_(f), givenby ##EQU1## where T_(e), is the electron temperature, m_(i) is the ionmass and m_(e) is the electron mass. For a hydrogen plasma at a typicaltemperature of 2eV, V_(f) ≈5.7V; in a water plasma, where the principalion is H₃ O⁺, V_(f) ≈8.6 V. Ions accelerated to the walls by thesepotentials will have little or no deleterious effect on the depositionprocess. On the other hand, these potentials are sufficient to sustainunipolar arcs on the chamber walls and these can be highly deleteriousto any process because they introduce significant quantities of wallmaterial into the plasma. There appears to be evidence that unipolararcs are occurring in the 3M2 reactor.

Unipolar arcs can occur in metal chambers whenever V_(f) >V_(c), whereV_(c) is the cathode fall of an arc. V_(c) depends on the cathodematerial and is in the range 7-12 V. Further details may be had byreference to the paper by A. D. Robson and P. C. Thonemeann, entitled"An Arc Maintained on an Isolated Metal Plate Exposed to a Plasma,"Proc. Phys. Soc. (London) 73, 508, (1959), which is incorporated hereinby reference. Ways of eliminating unipolar arcs are discussed in thepaper by A. E. Robson and R. Hancox, "Choice of Materials and Problemsof Design of Heavy Current Toroidal Discharge Tubes," Proc. IEE (London)106A, Suppl. 2, 47 (1959). It seems unlikely that the methods describedin this paper, which is concerned with pulsed fusion systems, could beapplied to the unipolar arcs in 3M2. On the other hand, a method that isnot available in fusion systems is applicable to inductively coupledr.f. plasmas, namely: the direct control of the plasma floatingpotential V_(f), which is the subject of the present invention.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a method of eliminatingdeleterious and uncontrolled unipolar arcs in a metal-walled plasmachamber during deposition.

Another object of the invention is to provide an apparatus for emittinga controlled unipolar arc into the metal-walled plasma, therebyminimizing the introduction of impurities into the plasma during CVD dueto uncontrolled unipolar arcs.

SUMMARY OF THE INVENTION

These and other objects of the invention which will become apparenthereinafter are achieved by introducing a filament made of a refractorymetal, such as tantalum or tungsten, into a metal-walled plasma chamber,and heating the filament to produce a thermionic emission from thesurface of the filament into the plasma. This emission results incontrolled unipolar arcing at the wall of the plasma chamber. Thefilament is heated to a predetermined temperature by a combination ofheat input from the plasma and resistive heating by passing a currentthrough the filament from a power source.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1, the sole drawing figure, is a schematic, cross-sectional sideview of a slotted, metal-walled plasma chamber equipped with a filamentand heater, in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to understand the present invention, it is useful to considercertain physical properties of a plasma which develops in a metal-walledchamber. Thus the floating potential V_(f), as defined herein above,arises because the electrons in a plasma have greater velocity than theions, and tend to leave the plasma with a net negative charge. As isalso described herein above, a high V_(f) can result in arcing at thechamber wall, causing the above mentioned contamination problem. V_(f)accelerates ions to the wall but retards all but the fastest electronsin the generally Maxwellian distribution, so that in the steady statethe current of ions and the current of electrons leaving the plasma areequal. If a current I_(e) of electrons is introduced into the plasmafrom an independent source, the requirement for quasi-neutrality of theplasma will dictate that there should be a net electron current Ie tothe chamber walls and the floating potential reduces itselfautomatically to achieve this. If the total ion current to the wall(which in the steady state is equal to the total electron current to thewall) is Ii the analysis in Robson and Thonemann (loc. cit) can be usedto obtain:

    1n(1I.sub.e /I.sub.i)=(1-V/V.sub.f)(1n(m.sub.i /2π m.sub.e)/2)

where V is the new plasma wall potential when an electron current I_(e)is introduced into the plasma and V_(f) is the potential in the absenceof electron injection.

For a water plasma we have:

    1n(1+I.sub.e /I.sub.i)=4.31(1-V/V.sub.f)

while for a hydrogen plasma:

    1n(1+Ie/I.sub.i)=2.84(1-V/V.sub.f).

For example, it is found from numerical simulations that in a reactorrunning at 100 kW about 1% of the energy is conveyed to the chamberwalls by the kinetic and ionization energy of the ions (eV_(f) +eV_(i)).Typically, V_(f) +V_(i) ≈20 eV, so we may estimate I_(i) ≈50 A. In awater plasma with T_(e) =2 eV we have V_(f) =8.6 V. To reduce this to,for example, 5 V (at which unipolar arcs would almost certainly notexist) we require I_(e) ≈250 A.

In a hydrogen plasma with T_(e) =2 eV we have V_(f) =5.68 V and toreduce this to, for example, 4.5 V, we require I_(e) ≈40 A.

Referring now to FIG. 1, the sole drawing figure, a chemical vapordeposition apparatus, made in accordance with the invention, isillustrated, generally designated by the numeral 10. CVD apparatus 10comprises a metal-walled chamber 12 which is provided with a pluralityof slots 14. Slots 14 run between the interior and exterior surfaces ofchamber 10. An r. f. application coil 16 is wrapped around chamber 12. Aplasma is produced inside chamber 12, due to currents in coil 16, asdetailed herein above. A number of removable substrates 28 are disposedat the interior wall of chamber 12, to receive the deposited material,in the present case, a diamond material.

The plasma inside chamber 12 is subject to unipolar arcing, at theinside surface of metal-walled chamber 12. Filament or strip 18 isprovided extending into the interior of chamber 12 to eliminate theunipolar arcing. Filament 18 is made from a refractory metal, such astungsten or tantalum. Filament 18 passes through the wall of chamber 12by means of insulted feed through 20 and is connected by leads 24 to aheating power supply 22. To complete the circuit, heating supply 22 isconnected through line 26 to chamber 12.

For example, in operation of the above described apparatus, specificparameters are seen as follows: to inject a current of 50-200 A into the1000 kW plasma, filament 18 is introduced into the plasma and heated toabout 2800 K by a combination of heat input from the plasma andresistive heating by passing a current through it from external powersource 22. At this temperature, the thermionic emission from a tantalumsurface is, theoretically about 20 A/cm2. A more conservative estimateof current density is 10 A/cm². The required area of the tantalum stripis then 5-20 cm². The black-body radiation from the surface, whichdetermines the power required to heat the strip, is about 150 W/cm², fora total of 750-3,000 W.

The emitting strip is connected electrically by line 26 to the wall ofchamber 12 and the electron current from the strip 18 passes through theplasma and returns to the wall. The emitting strip 18 thus acts like acontrolled unipolar arc. Further details are available in Robson andThonemann (loc. cit). However, it is necessary to consider that theelectron emission from hot tantalum is accompanied by negligibleevaporation of the material, whereas the cathode spot of a unipolar arcon a cold metal surface is a copious source of evaporated aluminum.

It will be appreciated that the described apparatus is extremelyflexible in its design.

For example, a variety of filament material is possible, as well avariety of plasma chamber designs.

For these reasons, inter alia, will be appreciated that while preferredembodiments of the invention have been illustrated and described indetail herein, changes and additions may be had therein and theretowithout departing from the spirit of the invention. Reference should,accordingly, be had to the appended claim in determining the true scopeof the invention.

I claim:
 1. Apparatus for controlling the potential of a plasmaincluding a metal-walled chamber having an interior and an exterior, anda conductive coil adapted to carry a radio-frequency current wrappedaround the metal-walled chamber to produce a plasma within said chamber;comprising:a filament made of refractory metal and having a first end, asecond end, and a central portion formed in the shape of a probe, saidcentral portion of the filament extending into the interior of thechamber and said first and second ends of the filament passing through awall of the chamber to the exterior of the chamber; and a heating powersupply connected to said first and second ends of the filament and tothe chamber wall for heating said filament to a predeterminedtemperature above that of the plasma, producing thermionic emissionsfrom the filament to the plasma.